108 PHYSIOLOGY 



where they form the globin part of the haemoglobin molecule, or from 

 the leucocytes of the thymus gland, or from the spermatozoa of fishes. 

 The histones are precipitated from their watery solutions by addition 

 of ammonia, but are soluble in excess of this reagent. In the presence 

 of salts they are coagulated on boiling. With cold nitric acid they give 

 a precipitate which dissolves on warming, but is thrown down again on 

 cooling. The most characteristic feature of this class of bodies is, 

 however, the high proportion of diamino-acids and bases contained in 

 their molecule. 



(3) ALBUMINS. These are soluble in pure water and are pre- 

 cipitated by complete saturation with ammonium sulphate, zinc 

 sulphate, or sodio-magnesium sulphate. 



EGG ALBUMIN forms the greater part of the white of egg. It 

 gives the ordinary protein tests, coagulates on heating at about 

 75 C., and is precipitated from its solutions if shaken with a drop of 

 dilute acetic acid in excess of ether. It is laevo-rotatory, its specific 

 rotatory power being - - 35-5. 



SERUM ALBUMIN occurs in large quantities in the blood plasma, 

 serum, lymph, and tissue fluids of the body. It coagulates at 75 C., 

 and is distinguished from egg albumin by its greater specific rotatory 

 power, - - 56, and by the fact that it is not precipitated by ether and 

 sulphuric acid. Some vegetable proteins belong to this class, e.g. the 

 leuicosin of wheat. 



(4) GLOBULINS. These bodies are insoluble in pure water and 

 require the presence of a certain amount of neutral salt to dissolve 

 them. They are precipitated from their solutions by complete satura- 

 tion with magnesium sulphate or by half-saturation with ammonium 

 sulphate. The chief members of this class are : 



CRYSTALLIN, obtained from the crystalline lens by passing a 

 stream of carbon dioxide through an aqueous extract of this body. 



SERUM GLOBULIN or PARAGLOBULTN, a constituent of blood plasma 

 and blood serum. 



FIBRINOGEN, which occurs in blood plasma and is converted into 

 fibrin when the blood clots. 



PARAMYOSINOGEN, a normal constituent of muscle. 



Midway between these two groups may be placed the muscle 

 protein, myosin (or myosinogen), which, though soluble in pure water, 

 resembles the class of globulins in the ease with which it is precipitated 

 by the addition of neutral salts. 



In addition to the members of the globulins named above and 

 derived from the animal body, proteins allied to this class form an 

 important constituent of plants, and are found in large quantities in 

 many seeds used as articles of food. These are vegetable globulins. 

 Prominent members of the group are the edestins, which may be 



